1. Field of the Invention
This invention relates to communications networks. More particularly, this invention relates to methods and systems for improved signaling in communications networks configured as RPR rings and using MPLS techniques.
2. Description of the Related Art
The meanings of acronyms and certain terminology used herein are given in Table 1.
TABLE 1EROExplicit route objectFECForwarding equivalence classIETFInternet engineering task forceIFInterfaceIPInternet protocolIS-ISIntermediate Systems-Intermediate Systems Rout-ing ProtocolIS-IS-TEIS-IS enhancements for traffic engineeringLDPLabel distribution protocolLSPLabel-switched pathLSRLabel-switching routerMACMedia access controlMPLSMulti-protocol label switchingMPLS-TEMPLS traffic engineeringOSPFOpen Shortest path First. A routing protocolOSPF-TEOSPF enhancements used in traffic engineeringResv messageA message carrying a reservation request from areceiver to a senderRFCRequest for commentsRPRResilient packet rings - a protocolRRRecord routeRRORecord route objectRSVPResource reservation protocolRSVP-TEAn extension of RSVP, used in traffic engineeringSNMPSimple Network Management ProtocolSRPSpatial reuse protocolTETraffic engineeringTLVType-Length-Value. An encoding scheme
Multi-protocol label switching is a well-known method for transporting information formatted in multiple protocols using packets. The packets, when entering a MPLS system, are prefixed by one or more tags, called MPLS tags, which are followed by the original packet.
MPLS is described in detail by Rosen et al., in the IETF document, RFC-3031, entitled Multiprotocol Label Switching Architecture (January, 2001). This RFC, as well as other IETF REC's cited hereinbelow, is available on the Internet, or from the IETF Secretariat, c/o Corporation for National Research Initiatives, 1895 Preston White Drive, Suite 100, Reston, Va. 20191-5434, USA.
In conventional IP routing, each router along the path of a packet sent through the network analyzes the packet header and independently chooses the next hop for the packet by running a routing algorithm. In MPLS, however, each packet is assigned to a forwarding equivalence class (FEC) when it enters the network, depending on its destination address. The packet receives a short, fixed-length label identifying the FEC to which it belongs. All packets in a given FEC are passed through the network over the same path by label-switching routers (LSRs). Unlike IP routers, label-switching routers simply use the packet label as an index to a look-up table, which specifies the next hop on the path for each FEC and the label that the LSR should attach to the packet for the next hop.
Since the flow of packets along a label-switched path (LSP) under MPLS is completely specified by the label applied at the ingress node of the path, a LSP can be treated as a tunnel through the network. Such tunnels are particularly useful in network traffic engineering, as well as communication security. MPLS tunnels are established by “binding” a particular label, assigned at the ingress node to the network, to a particular FEC. Multiple tunnels may belong to the same FEC, but each tunnel will have its own label binding. In accordance with the conventions of IP networks, tunnels are necessarily unidirectional. In other words, duplex-tunneled communications between a pair of nodes at the edges of a network requires the establishment and binding of two separate, independent tunnels.
MPLS defines a label distribution protocol (LDP) as a set of procedures by which one LSR informs another of the meaning of labels used to forward traffic between and through them. Label distribution protocols are needed in order to set up and bind MPLS tunnels. One example of such a protocol is RSVP-TE, which is available as the IETF document RFC-3209, entitled RSVP-TE: Extensions to RSVP for LSP Tunnels. RSVP-TE provides several objects that extend the well-known Resource Reservation Protocol (RSVP), allowing the establishment of explicitly routed LSP's using RSVP as a signaling protocol. RSVP itself is described by Braden et al., in the IETF document RFC-2205, entitled Resource ReSerVation Protocol (RSVP)—Version 1 Functional Specification (September, 1997). Section 3.10 of this document provides for the definition of new objects and object classes to be used in RSVP signaling, such as those provided by RSVP-TE. Other signaling protocols for setting a LSP are given in the IETF documents RFC-3036, entitled LDP Specification, and RFC-3212, entitled Constraint-Based LSP Setup using LDP. 
LDP is used for hop-by-hop automatic generation of the LSP and is not relevant for traffic engineering applications. RSVP-TE and CR-LDP are both suited for MPLS-TE. MPLS-TE is described in the IETF document RFC-2702, entitled Requirements for Traffic Engineering Over MPLS, and includes the capability for selecting an explicit route for the LSP from the source node, adding bandwidth reservations for the LSP along the path, and setting up a protection mechanism.
MPLS and LSP techniques have been employed to some extent in networks having ring configurations. The leading bi-directional protocol for high-speed packet rings is the resilient packet rings (RPR) protocol, which is in the process of being defined as IEEE standard 802.17. Network-layer routing over RPR is described, for example, by Jogalekar et al., in IP over Resilient Packet Rings (Internet Draft draft-jogalekar-iporpr-00), and by Herrera et al., in A Framework for IP over Packet Transport Rings (Internet Draft draft-ietf-ipoptr-framework-00). A proposed solution for media access control (MAC—protocol layer 2) in bi-directional ring networks is the Spatial Reuse Protocol (SRP), which is described by Tsiang et al., in the IETF document RFC-2892, entitled The Cisco SRP MAC Layer Protocol. Using protocols such as these, each node in a ring network can communicate directly with all other nodes through either the inner or the outer ring, using the appropriate Media Access Control (MAC) addresses of the nodes. The terms “inner” and “outer” are used arbitrarily herein to distinguish the different ring traffic directions, as are the terms “east” and “west” and “clockwise” and “counterclockwise.” These terms have no physical meaning with respect to the actual configuration of the network.
An explicit route in MPLS is established by providing a list of hops within the LSP in a signaling message of a path signaling protocol, e.g. RSVP or CDR-LDP. Typically the protocol provides options to mandate or exclude particular hops, and to provide for “loose hops”, that is, hops in which first and last nodes are specified, but intermediate nodes are not of concern, and are not specified. In this context, the term hop includes any kind of IP address, and can specify a port within a node, or simply indicate the node generally. Interface IP addresses are used when a node has multiple interfaces, and the operator wishes to specify the interface through which the traffic is to pass.
In order to conserve IP addresses, it is recommended that point-to-point interfaces not be assigned an IP address. This is made possible by the use of unnumbered interfaces. The use of unnumbered interfaces for MPLS signaling is described in an IETF draft document Signalling Unnumbered Links in CR-LDP (draft-ietf-mpls-crldp-unnum-10. txt, and an IETF draft document Signalling Unnumbered Links in RSVP-TE (draft-ietf-mpls-rsvp-unnum-08. txt, both available on the Internet.
The ability to establish explicit routes when using MPLS together with RPR is an important function that is currently unavailable in the art. A RPR MAC on a node has two physical interfaces, “east” and “west”, but is assigned a single MAC address and therefore a single IP address. Nodes that are interconnected on a RPR ring behave as a multi-access network from the external perspective of an IP device. However, from the perspective of internal traffic flow, the RPR ring looks like a point-to-point network. In contrast, an Ethernet multi-access LAN appears as a multi-access network from both perspectives.